For much of the year, it is very slow. Rates are low and there are too many drivers for too little business. In the summer, weekend nights are good. If you are flexible on where you are moving to and want to drive for Uber, you might want to consider Boston. It is a much larger market with more work and better rates. Good luck!
What New Englander said. Most trips, outside of surge, pay exactly enough to break even. So if you like driving, meeting people, seeing where the night will take you that kind of thing Uber can be a fun hobby. Don't do it expecting any profits though with the exception of late night surges and major drinking holidays. Even without that you can occasionally get lucky with a long trip taking you where you were going anyway, but those profits get wiped out next time you make $3.20 after a long pickup.
Portland is a weird market. 90% of ride requests originate from a little 2 mile penninsula and are just going across the peninsula, meaning lots and lots of min-fares. Any trips that take you off the peninsula, result in dead miles because you'll want to drive back to the peninsula to catch your next ride. waiting for rides outside the peninsula can take a long time.